1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display panel, and more particularly to a VA (vertically aligned) or MVA (multi-domain vertically aligned) semi-transmissive liquid crystal display panel that offers high aperture ratio and brightness, and that offers good contrast without sacrificing the image quality due to crosstalk or the like.
2. Description of Related Art
In recent years, it has been becoming increasingly popular to use liquid crystal display devices not only in information communication devices but also in commonly used electric devices. A liquid crystal display device itself does not emit light, and hence a transmissive liquid crystal display device provided with a backlight is widely used. Disadvantageously, the backlight consumes a large amount of electric power. It is for this reason that, to reduce the electric power consumption, portable devices in particular use a reflective liquid crystal display device that requires no backlight. The problem here is that this reflective liquid crystal display device uses external light as its light source, and hence suffers from poor visibility in a poorly-lit room, for example. It is under this background that semi-transmissive liquid crystal display devices that offer transmissive and reflective displays have been eagerly developed in recent years.
A liquid crystal display panel used in this semi-transmissive liquid crystal display device has pixel regions, each having a transmissive portion provided with a pixel electrode and a reflective portion provided with both a pixel electrode and a reflecting layer. In a poorly-lit place, this liquid crystal display panel displays an image by means of the transmissive portion of the pixel region by turning on the backlight; in a well-lit place, it displays an image by means of the reflective region illuminated by external light without turning on the backlight. This advantageously eliminates the need to keep the backlight on all the time, making it possible to greatly reduce the electric power consumption.
Now, since, for example, the users of mobile devices typified by mobile phones or the like are limited, there has conventionally not been so much demand to use a liquid crystal display panel having a wide viewing angle in small display portions of such mobile devices. However, with mobile devices becoming more and more sophisticated these days, there is a huge surge in demand for mobile devices having a display portion provided with a liquid crystal display panel having a wide viewing angle. To satisfy this demand, instead of TN (twisted nematic) liquid crystal display panels that have been conventionally frequently used in mobile devices, VA or MVA semi-transmissive liquid crystal display panels have come to be developed increasingly eagerly (see JP-A-2003-167253 (claims, paragraphs [0050] to [0057], FIG. 1) and JP-A-2004-069767 (claims, paragraphs [0044] to [0053], FIG. 1)).
Here, an MVA semi-transmissive liquid crystal display panel disclosed in JP-A-2004-069767 will be described with reference to FIGS. 4A, 4B, and 5. FIG. 4A is a perspective view schematically showing the structure of an MVA semi-transmissive liquid crystal display panel 50. FIG. 4B is a diagram schematically showing in which direction liquid crystals are inclined when an electric field is applied to liquid crystals in a liquid crystal layer. FIG. 5 is a sectional view taken along line D-D shown in FIG. 4A.
In this semi-transmissive liquid crystal display panel 50, an inclined plane or a height difference 53 is formed between a reflective portion 51 and a transmissive portion 52, and the reflective portion 51 and the transmissive portion 52 run continuously through the height difference 53 laid in between. The semi-transmissive liquid crystal display panel 50 has a first substrate 54 and a pixel electrode 55 formed thereon, and the pixel electrode 55 includes a first opening region 56, where no pixel electrode 55 is formed. This first opening region 56 constitutes first alignment control means, and is formed so as to be astride the reflective portion 51 and the transmissive portion 52 with the height difference 53 laid in between.
As a result, a pixel electrode 55a formed in the reflective portion 51 and a pixel electrode 55b formed in the transmissive portion 52 are connected to each other via a single line 57 extending in the direction of the length of the semi-transmissive liquid crystal display panel 50.
In a common electrode 59 formed on a second substrate 58, second opening regions 60a and 60b are so formed respectively as to face the pixel electrode 55a formed in the reflective portion 51 and the pixel electrode 55b formed in the transmissive portion 52. These second opening regions 60a and 60b constitute second alignment control means. The second opening regions 60a and 60b are formed as cross-shaped slits, and are arranged in such a way that, in the vertical direction, the center of the second opening region 60a coincides with the center of the pixel electrode 55a and the center of the second opening region 60b coincides with the center of the pixel electrode 55b. 
In this semi-transmissive liquid crystal display panel 50, when no electric field is applied between the pixel electrode 55 and the common electrode 59, liquid crystal molecules 61 in the liquid crystal layer are aligned with their long axes perpendicular to the surfaces of the pixel electrode 55 and the common electrode 59, blocking the passage of light. When an electric field is applied to the liquid crystal molecules 61 in the liquid crystal layer, as shown in FIGS. 4B and 5, the ends of the liquid crystal molecules 61, the ends being located on the side of the common electrode 59, are inclined toward the line 57 above the first opening region 56 in the height difference 53, and are inclined toward the center of the reflective portion 51 above the reflective portion 51 and toward the center of the transmissive portion 52 above the transmissive portion 52. This allows the light to pass through. As described above, according to the semi-transmissive liquid crystal display panel 50, the liquid crystal molecules are aligned in a given direction, making it possible to reduce degradation in visual characteristics or response speed.
In the MVA semi-transmissive liquid crystal display panel 50 described above, the height difference 53 is formed between the reflective portion 51 and the transmissive portion 52, which are located on the first substrate 54 side, and thereby, as is well known, a cell gap d1 in the reflective portion 51 and a cell gap d2 in the transmissive portion have the relationship d1=(d2)/2. In this way, adjustment is performed so that the image quality in the reflective portion 51 and the image quality in the transmissive portion 52 are made equal to each other. Such a cell gap adjustment can be performed on the second substrate 58 side, as practiced in another conventionally known type of MVA semi-transmissive liquid crystal display panel.
As another conventional example, an MVA semi-transmissive liquid crystal display panel having a topcoat layer for a cell gap adjustment formed on a second substrate side will be described with reference to FIGS. 6 to 8. FIG. 6 is a plan view showing one pixel of a conventional semi-transmissive liquid crystal display panel having a topcoat layer for a cell gap adjustment formed on a second substrate side, as seen through the second substrate. FIG. 7 is a sectional view taken along line E-E shown in FIG. 6. FIGS. 8A and 8B are sectional views, in which a second substrate is not shown, taken along line F-F and line G-G, respectively, shown in FIG. 6.
In a semi-transmissive liquid crystal display panel 70, a plurality of scan lines 12 and signal lines 13 are arranged so as to form a matrix, directly or via an inorganic insulating film 14, on an insulating transparent glass substrate 11 serving as a first substrate. Here, an area enclosed by the scan and signal lines 12 and 13 corresponds to one pixel, each pixel has a thin-film transistor TFT serving as a switching element, and the surface of the TFT, for example, is coated with a protective insulating film 23.
In a reflective portion 15 and a transmissive portion 16, an interlayer film 17 is laid on top of the scan lines 12, the signal lines 13, the inorganic insulating film 14, and the protective insulating film 23, for example. In the reflective portion 15, the interlayer film 17 is formed of an organic insulating film having fine projections and depressions on the surface thereof, in the transmissive portion 16, the interlayer film 17 is formed of an organic insulating film having a flat surface. Note that, in FIGS. 6 and 7, the projections and depressions formed in the reflective portion 15 are not shown. The interlayer film 17 has a contact hole 20 in a part thereof that corresponds to the drain electrode D of the TFT. In each pixel, the reflective portion 15 has a reflecting layer 18 made of aluminum, for example, formed on the surface of the interlayer film 17. On the surface of this reflecting layer 18 and the surface of the interlayer film 17 formed in the transmissive portion 16, a transparent pixel electrode 19 made of ITO or IZO, for example, is formed.
In the reflective portion 15, an auxiliary capacity line 21 is disposed below the reflecting layer 18 formed on the surface of the interlayer film 17, and the reflecting layer 18 and the pixel electrode 19 are formed in such a way that, as seen in a plan view, they do not abut on a reflecting layer and a pixel electrode of an adjacent pixel and that they slightly overlap the scan line 12 and the signal line 13 for preventing light leakage. Likewise, in the transmissive portion 16, the pixel electrode 19 is formed in such a way that, as seen in a plan view, it does not abut on a pixel electrode and a reflecting layer of an adjacent pixel and that it slightly overlap the scan line 12 and the signal line 13.
In this semi-transmissive liquid crystal display panel 70, a slit 33 is formed in the pixel electrode 19 for controlling the alignment of liquid crystal molecules along the boundary between the reflective portion 15 and the transmissive portion 16. As a result, the pixel electrode 19 is practically divided into two regions: one of which is a pixel electrode 19a formed in the reflective portion 15 and the other is a pixel electrode 19b formed in the transmissive portion 16. The pixel electrode 19a formed in the reflective portion 15 and the pixel electrode 19b formed in the transmissive portion 16 are electrically connected to each other via a smaller-width portion 34. A vertical alignment film (unillustrated) is laid on the surface of the pixel electrode 19 in such a way that all the pixels are coated therewith.
On the other hand, on the display region of an insulating transparent glass substrate 25 serving as a second substrate, a stripe-shaped color filter layer 26 having a color corresponding to each pixel, that is, one of three colors: red (R), green (G), or blue (B), is formed. Here, the thickness of the color filter layer 26 is uniform in the reflective portion 15 and the transmissive portion 16, and the color filter layer 26 has a topcoat layer 27 having a predetermined thickness in a part thereof that corresponds to the reflective portion 15. The topcoat layer 27 is formed over the entire length and breadth of the reflective portion 15, and the thickness thereof is adjusted so that the thickness of a layer of liquid crystals 29 in the reflective portion 15, i.e., the cell gap d1 is half the thickness of the cell gap d2 in the transmissive portion 16, that is, d1=(d2)/2.
In addition, protrusions 31 and 32 for controlling the alignment of the liquid crystals are formed respectively on the part of the surface of the color filter layer 26 located in the transmissive portion 16 and on the part of the surface of the topcoat layer 27 located in the reflective portion 15. On the surfaces of the color filter layer 26, the topcoat layer 27, and the protrusions 31 and 32, a common electrode (unillustrated) and a vertical alignment film (unillustrated) are laid on top of another.
The first substrate and the second substrate are then located face-to-face, and then bonded together by means of a sealing member provided around them. Then, a space between the substrates is filled with liquid crystals 29 with negative dielectric anisotropy. In this way, the MVA semi-transmissive liquid crystal display panel 70 is obtained. Although not shown in the figure, a conventionally known backlight provided with a light source, a light guide plate, and a diffusing sheet, for example, is disposed below the first substrate.
In the MVA semi-transmissive liquid crystal display panel 70 described above, when no electric field is applied between the pixel electrode 19 and the common electrode, the liquid crystal molecules in the liquid crystal layer are aligned with their long axes perpendicular to the surfaces of the pixel electrode and the common electrode, blocking the passage of light; when an electric field is applied between the pixel electrode and the common electrode, the light is allowed to pass through. This reduces the influence of light leakage occurring in the transmissive portion on the image quality. Furthermore, the presence of the slit 33 formed in the pixel electrode 19 and the protrusions 31 and 32 makes the liquid crystal molecules inclined toward the protrusion 31 or 32, greatly improving the viewing angle.
In the MVA semi-transmissive liquid crystal display panel 70 described above, to prevent light leakage, the pixel electrode 19b is formed in the transmissive portion 16 in such a way that it does not abut on a pixel electrode and a reflecting layer of an adjacent pixel and that it slightly overlaps the scan line 12 and the signal line 13. Likewise, the reflecting layer 18 and the pixel electrode 19a are formed in the reflective portion 15 in such a way that they do not abut on a reflecting layer and a pixel electrode of an adjacent pixel and that they slightly overlap the scan line 12 and the signal line 13 for preventing light leakage.
As described above, when the pixel electrode 19 is formed along the signal line 13 so as to overlap it as seen in a plan view, capacitance Csd is produced between the signal line 13 and the pixel electrode 19 as shown in FIGS. 8A and 8B, which are sectional views taken along line F-F and line G-G, respectively, shown in FIG. 6. As seen in FIG. 9 showing an equivalent circuit of one pixel, the capacitance Csd is parasitic capacitance produced in parallel between the drain and source electrodes of the TFT. When the capacitance Csd is equal to or greater than a predetermined value, crosstalk occurs on the display screen as the liquid crystal display device is driven. Thus, by reducing the capacitance Csd, that is, by reducing the width of the space where the signal line and the pixel electrode overlap one another, it is possible to reduce crosstalk.
Now, in the MVA semi-transmissive liquid crystal display panel 70 described above, when no electric field is applied between the pixel electrode 19 and the common electrode, the liquid crystal molecules in the liquid crystal layer are aligned with their long axes perpendicular to the surfaces of the pixel electrode and the common electrode, blocking the passage of light. This should prevent leakage of light. Thus, to reduce the capacitance Csd produced between the signal line 13 and the pixel electrode 19 and furthermore to obtain a semi-transmissive liquid crystal display panel with high aperture ratio, the pixel electrode 19 can be so formed along the scan line 12 and the signal line 13 as not to overlap them as seen in a plan view.
However, the inventors of the present invention have found out by experiments that, when the pixel electrode 19 is so formed along the scan line 12 and the signal line 13 as not to overlap them as seen in a plan view, light leaks between the pixel electrode 19b formed in the transmissive portion 16 and the scan line 12, leading to reduction in contrast in a display image.
The reason is considered to be as follows. Scanning signals are sequentially inputted to the scan line 12 frame by frame, and, even when no scanning signal is inputted thereto, a given voltage is normally applied to the scan line 12. At this time, since the scan line 12 is exposed as seen in a plan view, a potential difference is produced between the scan line 12 and the common electrode. This makes the liquid crystal molecules existing near the scan line 12 inclined all the time, resulting in light leakage. Similar problems arise in a VA semi-transmissive liquid crystal display panel that is provided with a pixel electrode having no slit and a second substrate having no alignment control means for controlling the alignment of liquid crystals.